The present invention relates in general to object locating and tracking systems, including those of the type described in U.S. Patent to Belcher et al, U.S. Pat. Nos. 5,920,287, and 5,995,046, assigned to the assignee of the present application and the disclosures of which are incorporated herein, and is particularly directed to the generation of a rotating AC magnetic field that effectively maximizes magnetic field coupling with the magnetic field sensor of an RF burst-transmitting tag that comes within a prescribed proximity of a (tag-programming) magnetic field generator.
The general architecture of the radio tagged object location systems described in the above-referenced ""287 and ""046 Patents is diagrammatically shown in FIG. 1 as comprising a plurality of tag emission readers 10 geographically distributed within and/or around an asset management environment 12. This environment contains a plurality of objects/assets 14, whose locations are to be monitored on a continuous basis and reported to an asset management database 20, which is accessible by way of a computer workstation or personal computer 26. Each of the tag emission readers 10 monitors the asset management environment for RF emissions from one or more RF-transmitter-containing tags 16 that are affixed to the objects 14. Each tag""s transmitter is configured to repeatedly transmit or xe2x80x98blinkxe2x80x99 a very short duration, wideband (spread spectrum) pulse of RF energy, that is encoded with the identification of the object and other information that may be stored in a tag memory.
These blinks or bursts of RF energy emitted by the tags are monitored by the readers 10, which are installed at fixed, and relatively unobtrusive locations within and/or around the perimeter of the environment being monitored, such as doorway jams, ceiling support structures, and the like. The output of each tag reader 10 is coupled to an associated reader processor. The reader processor correlates the spread spectrum RF signals received from a tag with a set of spread spectrum reference signal patterns, to determine which spread spectrum signals received by the reader is a first-to-arrive RF spread spectrum signal burst transmitted from the tag.
The first-to-arrive signals extracted by the reader output processors are forwarded to an object location processor within the processing subsystem 24. Using time-of-arrival differentiation of the detected first-to-arrive transmissions, the object location processor executes a prescribed multilateration algorithm to locate (within a prescribed spatial resolution (e.g., on the order of ten feet) the tagged object of interest.
In their normal mode of use, the tags exhibit a prescribed operational functionality, such as transmitting or xe2x80x98blinkingxe2x80x99 an RF signal at a relatively slow repetition rate. The use of a relatively slow blink rate is due to the fact that most of the objects being tracked do not move frequently. However, there may be occasions where it is desired to change the operation of or otherwise communicate information to a tag, such as stopping the tag from blinking or causing it to start blinking, or to transmit additional data, such as that acquired from optional sensors or a data bus.
As another illustration, there are times when the objects to which the tags are attached are moved and may pass through one or more regions of the monitored environment where communications with the tags are desired. For example, the monitored environment may contain xe2x80x98increased sensitivityxe2x80x99 regions (such as doorways and the like) where more frequent tag transmissions are desired, in order to ensure that any objects passing therethrough will be readily tracked. One way to accomplish this particular task would be to simply program the tags to blink more frequently on a continuous basis. However, this approach is not acceptable for two reasons. First, more frequent tag transmissions on a continuous basis will shorten the battery life of the tag; secondly it would increase spectrum congestion.
In accordance with the invention disclosed in the above-identified ""290 and ""079 applications, the above-described tag-reprogramming function is readily achieved by placing an arrangement of one or more relatively short range, modulated magnetic field proximity-based, tag-programming generators or xe2x80x98pingersxe2x80x99 at a respective location of the monitored environment that is proximate to a region (such as a doorway) through which a tag may pass. This tag-programming pinger arrangement is operative to emit a non-propagating, AC magnetic field, that is sensed by the tag and used to controllably prompt (or program) the tag to take some action. As a non-limiting example, the tag reprogramming field may be used to cause the tag to immediately begin blinking at an increased rate for a relatively brief period of time, so as to alert the tag-tracking system of the presence of the tag in the region of interest.
Pursuant to the tag-programming scheme disclosed in the ""290 application, the magnetic field is modulated or encoded with frequency shift keyed (FSK) encoding signals representative of digital data to be transmitted to the tag, using an FSK-encoded magnetic field based communication scheme of the type detailed in each of the ""290 and ""340 applications. The use of an FSK-encoded AC magnetic field using operational frequencies typically less than a few hundred KHz allows a large amount of data to be rapidly communicated to the tag.
An alternative approach is detailed in the ""079 application, which has particular utility in an environment (such as an industrial facility), where such frequencies may be dominated by man-made and natural noise levels that limit performance, and where the function to be carried out may be relatively simple. In this alternative scheme, rather than use an FSK-encoded magnetic field communication link to transmit xe2x80x98dataxe2x80x99 (where a fairly large (data) bandwidth may be required and increases susceptibility to interference), the programming magnetic field communication link employs a single frequency or is unmodulated. An example of such a simple function includes proximity detection or transmitting a relatively simple command, which considerably reduces the bandwidth requirement, and allows a reduced complexity communication implementation.
Irrespective of the type of AC magnetic field generation approach employed (modulated or nonmodulated), what is essential to successful operation of the magnetic field communication system is the absence of xe2x80x98orientation nullsxe2x80x99 in the magnetic field coverage for the xe2x80x98increased sensitivityxe2x80x99 region (such as a doorway and the like), so as to effectively guarantee than any tag entering that region will be fully operationally magnetically coupled with the generated field. These orientation nulls result from misalignment of the magnetic field transmit and receiver coil axes. Indeed, turning the axis of either coil orthogonal to that of peak performance can result in zero-coupling (an orientation null).
A reasonably acceptable resistance to this orientation null effect when the receiver coil lies in a given or fixed orientation can be achieved by sequentially shifting the vector orientation of the transmit field generator over three orthogonal axes. If the magnetic field receive system is comprised of two independent coils operating in such xe2x80x98polarization diversityxe2x80x99 (for example, the two receive coils are mutually orthogonal), the coupling performance is essentially coil orientation independent.
Previous attempts to solve the orientation null problem have involved sequentially generating a set of magnetic fields having three respectively different orientations. Unfortunately, this causes a substantial reduction in the rate at which information can be transmitted, since the same data must be repeated three times (one for each orientation).
In accordance with the present invention, the desire to effectively eliminate such an orientation null is successfully accomplished by a combination of the diverse spatial orientation of a plurality of magnetic field coils, and the driving of those coils in a prescribed phase relationship (e.g., in phase quadrature for spatially orthogonal coils) to realize a composite AC magnetic field that rotates (at the carrier frequency of the coil drive signal) over the entirety of the spatial coverage area.
From a practical standpoint, the spatial coverage area of the increased sensitivity region of a typical monitored environment, such as an exit or entry doorway, through which a tag may pass will customarily be two dimensional, making a two-dimensionally arranged coverage system both practical and cost effective. Moreover, the AC magnetic field produced by each generator""s field coil is three-dimensional, and therefore includes a magnetic field component orthogonal to the plane of the rotating composite field vector.
The net result is that the rotating field coverage produced by a set of two-dimensionally orthogonally oriented phase offset generators provides on the order of better than ninety-nine percent coverage, regardless of tag orientation. If desired, the invention may be used to realize a three-dimensional or volumetric coverage, by distributing at least three coils along three mutually orthogonal coordinate axes relative to the monitored volume of interest, and driving the coils at respectively different phase offsets.
To produce an AC magnetic field with a two-dimensional rotation, a pair of magnetic field generator subsystems are spatially arranged such that the axes of their output coils are effectively coplanar with the two dimensional coverage area, and such that their fields are spatially orthogonal to and overlap one another in the coplanar region. In addition, for a given excitation AC frequency, the carrier drive signal applied to one magnetic field generator is offset in phase by a prescribed differential, e.g., a quarter of a cycle (or ninety degrees) relative to the carrier frequency for the other (spatially orthogonal) magnetic field generator.
For an equal amplitude drive signal and phase quadrature difference in phase, for identical generators, the resultant AC magnetic field produced will exhibit a generally circular rotation within the two dimensional coverage plane at the applied carrier frequency (e.g., on the order of 100 KHz). It has been found that circular coverage provided by such an orthogonally oriented of phase offset generators provides on the order of better than ninety-nine percent coverage, regardless of tag orientation.
In an alternative configuration, the phase of the coil drive signals may be other than ninety degrees and/or the coils may be spatially non-orthogonal. This would enable the shape of the resulting composite AC magnetic field to have an elliptical rather than a circular rotation.
If the invention is used to realize three-dimensional rotation of the AC magnetic field, three sets of coils are distributed along three mutually orthogonal coordinate axes adjacent to the monitored volume of interest. These spatially orthogonal coils are driven by three respective magnetic field generator arrangements, so that the three orthogonal AC magnetic fields produced thereby are spatially orthogonal to and overlap one another in the coplanar region. As in the two-dimensional system, for a given excitation AC frequency, the drive signal applied to a respective magnetic field generator subsystem is offset by a quarter of a cycle (or ninety degrees) relative to the drive signal for the other two (spatially orthogonal) magnetic field generator subsystems. As a result, for an equal amplitude drive signal for substantially identical generators, the composite AC magnetic field produced by the three orthogonal field generator subsystems will exhibit a generally spherical rotation within the coverage volume at the applied carrier frequency.